1. Field of Invention
This invention relates generally to systems and methods for determining headings, positions, and velocities of moving vessels. The invention also relates to ship docking systems. In particular, it pertains to a system and method for determining position, attitude, and velocity of ships that can be used in a ship docking system.
2. Description of Related Art
Conventional docking aid systems can generally be grouped into two categories: those based on radar technology, and those based on laser technology. Some systems may even operate in dual mode. Both types of systems install sensors near the dock to measure ship navigation information such as distance to the dock and approach speed. This information is then made available to a pilot onboard the ship. Since lasers and radar have to be installed and operated on the dock, they can not easily be made portable. Also, since the operational range is rather limited, a large number of such systems must be installed for each port, making wide use of such aiding devices prohibitively expensive.
The declaration of Full Operational Capability (FOC) of the Global Positioning System (GPS) by the U.S. Air Force in July 1995 marked the beginning of a new era in navigation technology. The U.S. Coast Guard has upgraded more than 50 existing radiobeacons with the capability of broadcasting differential GPS (DGPS) corrections. With appropriate equipment, mariners inside the coverage areas can achieve 5 to 10 meters absolute positioning accuracy. Indeed, the DGPS-based navigation aid has been widely used in various marine navigation applications from open ocean to harbor approach. However, the existing DGPS systems cannot be used directly for a ship docking application because the ship's heading, which is critical in the final docking phase, cannot be determined with conventional DGPS methods.
To partially ameliorate the DGPS deficiency in heading information, a heading sensor may be integrated with a DGPS system. Unfortunately, conventional heading sensors, such as magnetic compasses, gyrocompasses, and fluxgates, either do not provide the accuracy required or need a lengthy alignment process before operation. None of these sensors can meet the requirements of precise accuracy and easy installation. Nor can a sufficient level of accuracy be achieved by differential operation alone, which only processes pseudorange measurements.